Electrodes for electrochemical processes

ABSTRACT

An electrode for use in electrochemical processes is manufactured by applying to a film-forming metal support member (preferably a titanium member) first a layer of an operative electrode material (preferably a layer of ruthenium dioxide) and then a layer of a film-forming metal oxide (preferably a layer of titanium dioxide by thermal decomposition of a coating comprising an organo-titanium compound) and optionally repeating this twolayer-application sequence at least once.

United States Patent 1 Scrutton et al.

[ ELECTRODES FOR ELECTROCHEMICAL PROCESSES [75] Inventors: AnthonyScrutton; Denis Lee, both of Runcorn, England [73] Assignee: ImperialChemical Industries Limited, London, England [22] Filed: Mar. 1, 1971[21] Appl. No.: 119,892

[30] Foreign Application Priority Data [451 Nov. 20, 1973 [56]References Cited UNITED STATES PATENTS 3,663,280 5/1972 Lee 204/290 FPrimary Examiner-Cameron K. Weiffenbach Att0rney--Cushman, Darby &Cushman [5 7 ABSTRACT An electrode for use in electrochemical processesis manufactured by applying to a film-forming metal support member(preferably a titanium member) first a layer of an operative electrodematerial (preferably a layer of ruthenium dioxide) and then a layer of afilmforming metal oxide (preferably a layer of titanium dioxide bythermal decomposition of a coating comprising an organo-titaniumcompound) and optionally repeating this two-layer-application sequenceat least once.

7 Claims, No Drawings ELECTRODES FOR ELECTROCHEMICAL PROCESSES Thisinvention relates to electrodes for electrochemical processes. Moreparticularly it relates to improvements in the durability of electrodescomprising a layer of operative electrode material on a support formedfrom a film-forming metal, especially titanium.

It is known to employ as an anode in an electrochemical cell,particularly in a cell wherein an aqueous solution of an alkali metalchloride is electrolysed, an electrode comprising a film-forming metalsupport, particularly a titanium support, which carries on at least apart of its surface a coating of an operative electrode material. Thetitanium support is resistant to anodic attack, even in the highlycorrosive chloride electrolytes, but is incapable of operating as ananode since the thin film of oxide which is formed on its surface actsas a barrier layer preventing the passage of current directly into theelectrolyte. The operative electrode material must be resistant toanodic attack and must also be active in transferring electrons to theelectrode from the ions of the electrolyte, i.e., in distinction fromthe film-forming metal of the support it must be capable of operating asan electrode when immersed in the electrolyte. The operative electrodematerial is usually one or more of the platinum group metals and/or theoxides of these metals, but it may be any other electroconductingmaterial which has adequate resistance to anodic dissolution in the celland which will function as an anode.

Although the above-mentioned operative electrode materials are veryresistant to electrochemical attack in a variety of corrosive media,they do wear away at an appreciable rate or even break away from thefilmforming metal support in use. The present invention provides amethod of anchoring the operative electrode material more securely tothe support in an electrode of the aforesaid type while at the same timepresenting the operative electrode surface in a form which has a lowoverpotential for the liberation of chlorine when the electrode is usedas an anode in the electrolysis of alkali metal chloride solutions.Furthermore, electrodes prepared according to the invention have a highresistance to damage by short-circuit contact with the cathode when theyare used as anodes in mercurycathode cells.

The improved electrodes of the invention are manufactured by firstpreparing an electrode of the type comprising a film-forming metalsupport carrying a coating of an operative electrode material by amethod as known in the art, or by a simple variant thereof as willappear hereinafter, then applying over the layer of operative electrodematerial a coating of a thermally decomposable organo-compound of afilm-forming metal and heating the thus coated electrode so as toconvert the organo'compound of the film-forming metal to an oxide of thefilm-forming metal. The consecutive steps of coating with an operativeelectrode material and then with an oxide ofa film-forming metal may berepeated at least once more so as to lay down a sequence of alternatinglayers of operative electrode material and film-forming metal oxideamounting to at least four layers in all, with advantages as will appearhereinafter.

In its broadest aspect therefore the present invention provides a methodfor the manufacture of an electrode for use in electrochemicalprocesses, which comprises the steps of (l) applying to a support membermade of a film-forming metal a layer of an operative electrode material,and (2) applying over the said layer a coating comprising a thermallydecomposable organocompound of a film-forming metal in a liquid vehicleand heating the coating so as to convert the organocompound of thefilm-forming metal to an oxide of the film-forming metal.

In a preferred embodiment of the method of the invention the sequence ofthe steps (1) and (2) is repeated at least once so as to lay down asequence of alternating layers of operative electrode material andfilm-forming metal oxide on the support member. We have found that anelectrode prepared according to this embodiment has a further enhancedresistance to damage by short-circuit contact with the cathode when itis used as an anode in a mercury cell, as shown by the fact that aftercontact of the anode with the cathode in a working cell the cell voltagerises more slowly with time and there is a less rapid loss of anodecoating from the film-forming metal support. Preferably the sequence ofsteps (1) and (2) is carried out a sufficient number of times to producea total coating of operative electrode material and film-forming metaloxide on the filmforming metal support in the range 10-15 g/m of thecoated surface.

The finished coating has a smooth vitreous appearance and excellentadhesion to the metal of the support. The two-step coating method of theinvention (with optional repetition of the two steps) enables thiscoating containing a film-forming metal oxide to be produced at atemperature below the melting point of the film-forming metal oxide andin fact at temperatures below the melting points of glasses that oxidesof film-forming metals form with other oxides. This is important sinceit enables high temperatures at which the metal of the support wouldreact appreciably with the coating or with oxygen of the atmosphere tobe avoided during the coating operation.

Although the electrode coatings have a smooth vitreous appearance it isapparent that the outermost layer of film-forming metal oxide producedby the method of the invention is not an impervious dielectric layer,since the electrodes are capable of carrying a high density of currentinto an electrolyte at low cell voltage.

The present invention also provides, therefore, an electrode for use inelectrochemical processes which comprises a support member made of afilm-forming metal carrying a coating which comprises a layer of anoperative electrode material and superimposed thereon a layer of anoxide of a film-forming metal.

A preferred form of the electrode comprises a support member made of afilm-forming metal having deposited thereon a multi-layer coatingconsisting of an alternating sequence of a layer of an operativeelectrode material and a layer of'a film-forming metal oxide, theinnermost layer being operative electrode material, the outermost layerbeing film-forming metal oxide and the total number of layers being atleast four.

Although the electrodes of the invention are produced by forming atleast two distinct coating layers on the film-forming metal supportfirst a layer of operative electrode material and afterwards afilm-forming metal oxide we do not exclude from the scope of theinvention electrodes in which there is some interpenetration between thedifferent coating layers. Indeed it seems likely that at thetemperatures employed to form the coating of film-forming metal oxide bydecomposition of an organo-compound of the metal some interdiffusion ofthe layers will take place, and this may be the explanation of the highmechanical strength of the electrodes of the invention.

In this specification, by a film-forming metal we mean one of the metalstitanium, zirconium, niobium, tantalum or tungsten or an alloyconsisting principally of one of these metals and having anodicpolarisation properties similar to those of the pure metal. It ispreferred to use as support material in the electrode titanium alone oran alloy based on titanium and having anodic polarisation propertiessimilar to those of titanium. Examples of such alloys aretitanium-zirconium alloys containing up to l4 percent of zirconium,alloys of titanium with up to 5 percent of a platinum metal such asplatinum, rhodium or iridium and alloys of titanium with niobium ortantalum containing up to percent of the alloying constituent.

The operative electrode material of the electrode may be one or more ofthe platinum group metals, i.e., platinum, rhodium, iridium, ruthenium,osmium and palladium, and/or the oxides thereof, or another metal or acompound which is resistant to electrochemical dissolution in the cellwhere it is to be employed and will function as an electrode, forinstance rhenium, rhenium trioxide, manganese dioxide, magnetite,titanium nitride and the borides, phosphides and silicides of theplatinum group metals. The preferred operative electrode materials arethe oxides of the platinum group metals, particularly ruthenium dioxide,and mixtures of one or more platinum group metals with the oxidesthereof. The invention will be further described by reference to the useof these preferred operative electrode materials but is not limitedthereto.

The method employed for forming a layer of the platinum group metaloxide or a mixture of a platinum group metal and the oxides thereof on asupport of a film-forming metal may suitably be one of those known inthe art. The same methods may suitably be employed for depositingfurther layers of the platinum group metals and/or oxides onfilm-forming metal oxide layers when building up multi-layer coatings.According to the British Patent Specification No. 984,973 for example, acoating of a platinum metal may be formed on a titanium support byapplying to the chemically-cleaned support a plurality of coatings of aplatinum-bearing preparation comprising a platinum metal compound in anorganic vehicle and containing a reducing agent, e.g., an essential oil,and heating each coating in an oxidising atmosphere, e.g., air, at atemperature between 350 and 550C. The platinum metal compounds may bethermally-decomposable inorganic compounds, resinates or sulphoresinatesof the platinum metals. It can be shown that coatings produced in thismanner contain at least a proportion of the platinum metal in the formof its oxides and that with the more easily oxidised platinum metals,e.g., ruthenium, and a firing temperature in the upper half ofthe saidtemperature range, the resultant coating consists substantially of theoxides of the platinum metal.

The method .of the aforesaid British specification is suitable for usewith the present invention. The method may, however, be varied ifdesired by heating each coating of the platinum-bearing preparationfirst at a lower temperature to reduce the platinum metal compounds andproduce a layer consisting substantially of the platinum metal and thenin an oxidising atmosphere at a higher temperature, suitably at least350C, to convert the platinum metal at least in part to its oxides. Forexample a ruthenium chloride coating composition containing a reducingagent may first be heated at approximately 300C to reduce the chloridesubstantially to ruthenium metal and the ruthenium coating may then beconverted substantially completely to ruthenium dioxide by heating inair at approximately 450C.

For the purpose of the present invention a coating of the oxides of oneor more platinum metals on a filmforming metal support may also beformed directly from thermally decomposable compounds of the platinummetals, i.e., without intermediate reduction of the metals, by coatingthe support with a composition comprising compounds of the platinummetals and an organic vehicle but containing no reducing agent andheating in an oxidising atmosphere at a temperature higher than 300C,preferably at least 350C and most preferably about 450C. For example, astaught in French Patent Specification No. 1,479,762, a coating ofpalladium oxide is formed directly on a film-forming metal support,e.g., titanium, by heating a coating consisting of an acidified solutionof palladium chloride in isopropyl alcohol at 400-500C in an oxidisingatmosphere, e.g., air, and a coating of the mixed oxides of palladiumand iridium is formed directly on a tantalum support from a similarsolution of the chlorides of palladium and iridium by heating in air at300600C.

For the purpose of the present invention the preformed oxides of theplatinum metals may also be employed to produce the layer of operativeelectrode material. The preformed oxides may be applied to thefilm-forming metal support for example by any of the methods describedin the aforesaid French patent specification, for instance byapplication in the molten state, by coating with a dispersion of theoxide in a liquid vehicle or by electrophoresis on to the film-formingmetal support from a colloidal solution of the oxide. If desired toincrease the initial adhesion the platinum metal oxide coating may berolled or pressed into the support.

Although coatings consisting of platinum group metals at least in partin the form of their oxides are preferred, especially for electrodesthat are to be used as anodes under the arduous conditions ruling inmercurycathode cells electrolysing alkali metal chloride solutions, forless arduous conditions coatings consisting substantially of platinumgroup metals in the unoxidised state may be employed. Such coatings maybe prepared by thermal decomposition of compounds of platinum groupmetals under reducing conditions throughout, for instance by applying tothe support metal a solution ofa platinum group metal salt in an organicsolvent containing a reducing agent, e.g., linalool, and heating thecoating in an atmosphere of a gas having an alkaline reaction, e.g.,ammonia, and a reducing gas, e.g., methane, carbon monoxide, hydrogen ortown gas, as taught in British Patent Specification No. 964,913.

It should be understood that with any of the coating methods describedherein the coating steps may be repeated as necessary to build up adesired thickness of the operative electrode material before applyingthe film-forming metal oxide over it. Furthermore, when a final heatingstep in an oxidising atmosphere is employed to oxidise a coating ofplatinum group metal formed by thermal decomposition of a platinum groupmetal compound and the layer of operative electrode material is built upby superimposing a plurality of coatings, the oxidising step may becarried out in a single stage after all the coatings have been appliedor if desired, particularly when building up relatively thick layers,the oxidising step may be carried out on each coating before applyingthe next or each time after applying a fraction of the total number ofcoatings greater than one coating, for instance after each second orthird coating.

In general the coating of operative electrode material is applied to achemically cleaned surface of the filmforming metal support. The supportis degreased if necessary and then pickled, for instance inhot oxalicacid solution or in hot or cold hydrochloric acid. It is, however,possible to coat the operative electrode material on to a support whichhas been given an oxidising treatment, for instance by heating in air orby anodic treatment in an aqueous electrolyte, to produce a very thinsurface layer of the film-forming metal oxide after the aforesaidcleaning treatment, and this oxide layer may even be beneficial inproviding a better initial key for the operative electrode material, forinstance when this is a preformed platinum group metal oxide inparticulate form.

For use in the second coating step of the method according to theinvention the thermally decomposable organo-compound ofa film-formingmetal must be one that is decomposable by heat alone, optionally in anoxidising atmosphere, e.g., air, or by heating after partial hydrolysisas for instance by exposure to moisture in the atmosphere during thecoating operation, to form an oxide of the film-forming metal.Particularly suitable compounds are the alkyl titanates, alkylpolytitanates and alkyl halotitanates in which the halogen is chlorine,bromine or fluorine, and the corresponding compounds of the otherfilm-forming metals. The titanium compounds are preferred when thesupport metal of the electrode is titanium or a titanium alloy. Verysuitable compounds are those in which the alkyl groups contain two tofour carbon atoms each. Other suitable thermally-decomposable compoundsof the filmforming metals are, for example, resinates of the filmformingmetals, as may be made for instance by reaction of a halide of thefilm-forming metal with a resinous material, e.g., abietic acid, in anorganic solvent.

Methods of preparing alkyl titanates and preparing alkyl polytitanates(sometimes referred to as condensed alkyl titanates) by partialhydrolysis of alkyl titanates are disclosed in a paper by T. Boyd inJournal of Polymer Science, Vol. VII, No.6, 1951, pages 591-602. Analkyl chlorotitanate in the form of an alcoholic solution suitable foruse in the method of the invention may be prepared by heating titaniumtetrachloride with the chosen alcohol without employing any chemicalmeans to remove the hydrogen chloride formed from the reaction mixtureand using an excess of alcohol, suitably 2-5 times the amounttheoretically required to remove all the chlorine atoms from thetitanium tetrachloride. Alkyl bromotitanates and alkyl fluorotitanatesmay be prepared in similar manner starting with titanium tetrabromideand titanium tetrafluoride respectively.

The thermally-decomposable compound of a filmforming metal (exemplifiedhereinafter for simplicity by reference to alkyl titanates orhalotitanates) in a liquid vehicle, suitably a volatile alcoholicsolvent, may be applied by dipping, brushing or spraying on to thesurface of the layer of operative electrode material which haspreviously been formed on the film-forming metal support. The coating isthen suitably dried by heating in an oven at a moderate temperature,e.g., l00200C, to evaporate the solvent, after which the coatedelectrode is heated at a higher temperature, for instance 250800C, mostsuitably at approximately 450C, to convert the organo-compound of thefilmforming metal in the coating substantially to an oxide of the metal.Additional coats may be applied, dried and then decomposed by strongerheating in the same way, if necessary to obtain good coverage of theunderlying operation electrode material.

When alkyl titanates and halotitanates are applied in thin coatings, itappears that some condensation takes place by hydrolysis caused bymoisture in the atmosphere. When the condensed titanates are stronglyheated they decompose substantially completely to leave a residue oftitanium dioxide of vitreous appearance which serves to knit theoperative electrode mate rial securely to the underlying surface of thefilmforming metal support. The time of heating to decompose the titanateshould be shorter the higher the temperature employed, so as to avoidexcessive reaction between the film-forming metal support and thecoating or oxygen of the atmosphere. For example at a temperature of500C the time should not exceed about 15 minutes and at 800C it shouldnot exceed about 15 seconds. In this connection it should be noted thatwhen the operative electrode material contains or consists of a platinumgroup metal oxide, the final heating step to decompose the titanateshould be carried out in an oxidising atmosphere, e.g., air.

Electrodes produced according to the invention are useful inelectrolytic cells, electrodialysis cells, fuel cells and in cathodicprotection systems. Specific embodiments of these electrodes wherein thefilm-forming metal support is titanium and the coating on the titaniumcomprises one or more platinum group metal oxides and titanium dioxidehave special advantages when used as anodes in the electrolysis of analkali metal chloride solution.

The invention is further illustrated by the following working examplesin which all parts are by weight.

EXAMPLE I A strip of titanium was immersed overnight in hot oxalic acidsolution to etch the surface of the metal and was then washed and dried.A mixture containing ruthenium chloride 1 part, isopropyl alcohol 4parts and linalool 1.3 parts was painted on to the titanium, the coatingof paint was allowed to dry in air for 10 minutes and was then heated ina furnace in air at 300C for 10 minutes to form a coating consistingsubstantially of ruthenium. Two further coats of the paint were applied,dried and heated in the same manner. The rutheniumcoated titanium wasthen heated in air at 450C for 1 hour to oxidise at least the outermostpart of the ruthenium layer and was allowed to cool. A solution of ethylchlorotitanate in ethyl alcohol was prepared by heating 1 part oftitanium tetrachloride with 5 parts of absolute ethyl alcohol at C for15 minutes. Three coats of this solution were applied to the preparedtitanium strip over the ruthenium oxide coating, each coating beingdried in an oven at C for 10 minutes and then heated in air in a furnaceat 450C for 15 minutes to form a surface layer of smooth vitreousappearance.

EXAMPLE 2 A coating of ruthenium was formed on a strip of titanium andthe coating was then oxidised in air at 450C for 1 hour, all as inExample 1. The weight of the coating then amounted to about 6 g/m of thetitanium surface, calculated as ruthenium metal. Eight coats of asolution consisting of 5 parts of tetra-n-butyl titanate in 5 parts ofn-pentanol were then painted on to the coated titanium, each coat beingdried in an oven at 200C for minutes and then heated in air at 450C forminutes. The theoretical weight of titanium dioxide thus formed on theelectrode was g/m EXAMPLE 3 A ruthenium coating was formed on a strip oftitanium and oxidised in air at 450C, all as in Example l to give acoating amounting to about 6 g/m of the titanium surface, calculated asruthenium metal. A solution of isopropyl chlorotitanate in isopropylalcohol was prepared by heating 1 part of titanium tetrachloride with 5parts of isopropyl alcohol for 1 hour at 70C. Four coats of thissolution were then painted on to the coated titanium, each coat beingdried in an oven at 200C for 10 minutes and then heated in air in afurnace at 450C for 15 minutes. The theoretical weight of titaniumdioxide thus formed on the electrode was about 8 g/m EXAMPLE 4 A stripof titanium was etched, washed and dried as in Example 1 and was thencoated with the same ruthenium chloride paint composition as in thatExample, i.e., ruthenium chloride 1 part, isopropyl alcohol 4 parts,linalool 1.3 parts. Three coats of the paint composition were applied(equivalent coating weight about 6 g/m calculated as ruthenium metal)but this time after being allowed to dry for 10 minutes in air each coatwas heated once only, in air at 350C for 1 hour, to form a coatingconsisting substantially of ruthenium dioxide. Four coats of a solutionof isopropyl chlorotitanate were then applied and converted to titaniumdioxide as in Example 3.

EXAMPLE 5 A strip of titanium was pretreated and given three coats of aruthenium chloride paint composition as described in Example 4 exceptthat after being allowed to dry each coat was converted substantially toruthenium dioxide by heating once only, in air at 450C for 1 hour. Eightcoats of a solution of tetra-n-butyl titanate in npentanol were thenapplied, each coat being dried in an oven at 200C for 10 minutes andthen heated in air in a furnace at 450C for 15 minutes. The theoreticalweight of titanium dioxide thus formed on the electrode was 35 g/mEXAMPLE 6 A strip of titanium was etched, washed and dried as in Example1 and was then given three coats ofa ruthenium chloride paintcomposition containing no reducing agent (composition: rutheniumtrichloride 1 part, isopropyl alcohol 4 parts). Each coat was allowed todry in air for 10 minutes and was then heated in air at 350C for 1 hourto form a coating consisting substantially of ruthenium dioxide. Sixcoats of a solution of isopropyl chlorotitanate in isopropyl alcoholwere then applied to the coated titanium, each coat being dried in airin an oven at 200C and then heated in air in a furnace at 450C for 15minutes. The theoretical weight of titanium dioxide thus formed on theelectrode was 15 g/m EXAMPLE 7 A suspension of ruthenium dioxideparticles, mostly of size less than 4 microns diameter, in n-pentanolwas brushed on to a strip of titanium which had been etched, washed anddried as in Example 1 and the solvent was evaporated from the coating inan oven at 200C. Two further coats were applied and dried in the samemanner to give a total loading of 7g ruthenium dioxide/m of the coatedtitanium surface. A solution of tetra-n-butyl titanate in n-propylalcohol was then sprayed on to the ruthenium dioxide coating and thetitanate coating was dried in air in an oven for 10 minutes at 200C andthen heated in air in a furnace for 15 minutes at 450C. Seven furthercoats of the titanate solution were applied, each one being dried at200C and heated at 450C as for the first coat. The theoretical amount oftitanium dioxide thus formed on the electrode was 35 g/m Electrodesproduced according to each one of the foregoing Examples were tested asanodes in sodium chloride brine containing 21.5 percent NaCl by weightat 65C in an electrolysis cell with a mercury cathode, and at an anodiccurrent density of 8 kA/m they showed chlorine overpotentials in therange 25-76 mV. Each anode was also dipped into the sodium amalgamcathode with a potential of about 5 volts still applied between theanode and the amalgam. The short-circuit current was found to berelatively small less than 10 amps compared with an anode of the samesize consisting of titanium coated by firing thereon a coating of aplatinum-bearing preparation according to the prior art but with nofurther treatment, which passed a current of 400 amps under the sameshort-circuit conditions. This demonstrates one special advantage of anelectrode prepared according to the invention, in that such an electrodedevelops a resistance to short-circuit currents in the cell and providesits own protection against damage by accidental short-circuit in use.

EXAMPLE 8 A strip of titanium was immersed overnight in hot oxalic acidsolution to etch the surface of the metal and was then washed and dried.A ruthenium chloride paint composition was prepared by dissolvingovernight 2.5

parts of ruthenium chloride in 10 parts of n-pentanol and adding 3.25parts of linalool immediately before use. An alkyl titanate paintcomposition was made by mixing 10 parts of tetra-n-butyl titanate with10 parts of n-pentanol. A layer of ruthenium dioxide was deposited onthe titanium strip by spraying on a coating of the ruthenium chloridepaint, drying the coating in an oven at 180C for 10 minutes and thenfiring it by heating the coated strip in air in a furnace at 450C for 15minutes. Two thin layers of titanium dioxide were then superimposed onthe ruthenium dioxide layer, each one being producedby spraying on thealkyl titanate paint composition, then drying and firing the coating inthe same manner as the ruthenium chloride paint. The sequence ofdepositing one ruthenium dioxide layer followed by two titanium dioxidelayers was then twice repeated to give a total deposite on the titaniumsurface of approximately 6 g/m of ruthenium dioxide and 9 g/m oftitanium dioxide.

The electrode thus produced provided to have a high resistance to lossof the active coating when submitted to electrical shorting to a sodiumamalgam cathode in the test described with reference to the precedingExamples and it was still capable of operating as an anode at an anodiccurrent density of 8 kA/m without significant increase in cell voltageafter the short-circuiting test.

We claim:

1. A method for the manufacture of an electrode for use inelectrochemical processes, which comprises the steps of (1) applying toa support member made of a film-forming metal selected from the groupconsisting of titanium, zirconium, niobium, tantalum and tungsten or analloy consisting principally t one of these metals, a layer of anoperative electrode material resistant to electrochemical dissolution,and (2) applying over the said layer a coating comprising a thermallydecomposable organo-compound of a film-forming metal in a liquid vehicleand heating the coating so as to convert the organo-compound of thefilm-forming metal to an oxide of the film-forming metal, the sequenceof said steps (1) and (2) being repeated a plurality of times.

2. A method according to claim 1, wherein the total weight of operativeelectrode material and filmforming metal oxide deposited on thefilm-forming metal support member is in the range -15g/m of the coatedsurface.

3. A method according to claim 1, wherein each of the steps (1) consistsof a coating operation carried out a plurality of times to build up athickness of operative electrode material.

4. A method according to claim 1, wherein each of the steps (2) consistsof a plurality of said coating and heating operations whereby athickness of film-forming metal oxide is built up.

5. A method according to claim 1, wherein in each of the steps (1) theoperative electrode material is ruthenium dioxide, in each of the steps(2) the thermallydecomposable organo-compound of a film-forming metal isa resinate and the support member is made of titanium or an alloy basedon titanium and having anodic polarisation properties similar to thoseof titanium.

6. A method according to claim 1, wherein in each of the steps (1) theoperative electrode material is ruthenium dioxide, in each of the steps(2) the thermallydecomposable organo-compound of a film-forming metal isan alkyl titanate, an alkyl polytitanate or an alkyl halotitanate inwhich the halogen is chlorine, bromine or fluorine and the supportmember is made of ti tanium or an alloy based on titanium and havinganodic polarisation properties similar to those of titanium.

7. A method according to claim 6 wherein step (2) comprises drying thecoating by heating at l00200C and then heating at 250800C to convert theorganocompound to an oxide of the film-forming metal.

2. A method according to claim 1, wherein the total weight of operativeelectrode material and film-forming metal oxide deposited on thefilm-forming metal support member is in the range 10-15g/m2 of thecoated surface.
 3. A method according to claim 1, wherein each of thesteps (1) consists of a coating operation carried out a plurality oftimes to build up a thickness of operative electrode material.
 4. Amethod according to claim 1, wherein each of the steps (2) consists of aplurality of said coating and heating operations whereby a thickness offilm-forming metal oxide is built up.
 5. A method according to claim 1,wherein in each of the steps (1) the operative electrode material isruthenium dioxide, in each of the steps (2) the thermally-decomposableorgano-compound of a film-forming metal is a resinate and the supportmember is made of titanium or an alloy based on titanium and havinganodic polarisation properties similar to those of titanium.
 6. A methodaccording to claim 1, wherein in each of the steps (1) the operativeelectrode material is ruthenium dioxide, in each of the steps (2) thethermally-decomposable organo-compound of a film-forming metal is analkyl titanate, an alkyl polytitanate or an alkyl halotitanate in whichthe halogen is chlorine, bromine or fluorine and the support member ismade of titanium or an alloy based on titanium and having anodicpolarisation properties similar to those of titanium.
 7. A methodaccording to claim 6 wherein step (2) comprises drying the coating byheating at 100*-200*C and then heating at 250*-800*C to convert theorgano-compound to an oxide of the film-forming metal.